Diagnosing Eczema & Dermatitis

The different types of eczema and dermatitis can look and feel very similar on the skin. Rashes are often red, scaly, and dry but can also be cracked, oozing, and blistered. Most rashes are very itchy and people find it difficult not to scratch. 

NYU Langone dermatologists, with their extensive experience and expertise, may differentiate between atopic, contact, and nummular dermatitis simply by examining the distribution of the rash on your skin and asking questions about your family and medical history. 

Our doctors have also been leaders in the diagnosis of contact dermatitis since the 1930s, when the patch test was brought to the United States and the technique was refined. This allergy test is uniquely designed to identify the cause of contact dermatitis without using needles. The patch test remains the only reliable method of determining which substances cause an allergic reaction when they come into contact with the skin.

Physical Examination and Medical History 

A dermatologist carefully examines your skin during a physical exam. The pattern, location, and appearance of a rash provide our doctors with important information about its causes.

Your doctor may ask questions about when symptoms appeared, what parts of the body they affect, and whether a rash is persistent or comes and goes. They also want to know if there are any noticeable patterns about when the rash appears, such as if there is a seasonal variation or if the rash appears when using certain perfumes or after exposure to certain metals or fabrics. Knowing whether anyone else in your family has been diagnosed with eczema or dermatitis may help doctors better understand your diagnosis. 

Doctors may also ask about the personal hygiene products used in your household. Many cosmetics, moisturizers, and soaps contain irritating ingredients that may cause eczema and dermatitis. Our dermatologists can recommend nonirritating, fragrance-free products that have low levels of preservatives. Often, these are available at drugstores in a similar price range as the products you normally buy.

Patch Test

If dermatologists suspect that allergic dermatitis is causing your rash, a patch test is the most effective diagnostic tool. Usually, the test requires three visits to the doctor’s office over the course of one week. 

In the patch test, a number of thin metal or flexible plastic chambers are arranged in strips, or panels. Each chamber contains a small amount of an allergen. The allergens used in the test are found in products routinely used during everyday activities. Your doctor may recommend a series of chemicals based on the distribution of the rash and the specific substances you are exposed to at home or at work. 

The test panels are applied to the skin on your back and covered by tape. Your doctor keeps a record of the location of each allergen on your skin. The taped panels are left in place for around two days to allow the body to recognize and react to the possible allergens. You can follow most of your daily routine while wearing the tape panels, but you should avoid exercising, showering, and other activities that could get the patches wet.

After two days, the panels are removed and a note is made of any areas of irritation. A doctor may not record the final results for up to four days to allow for delayed reactions. On the final reading, our doctors conduct an examination to match signs of reaction on the skin with the substance that was placed there. Redness with elevated skin or a rash at the site of any of the tested allergens may help your dermatologist determine the source of the allergic reaction and confirm a diagnosis of allergic dermatitis.

Skin Biopsy

At times, a dermatologist needs to remove a small piece of skin for lab testing. This procedure, called a biopsy, is usually only necessary if doctors have not been able to diagnose your condition during a physical exam or patch test. 

A skin biopsy is a minor procedure performed in a doctor’s office. The doctor typically injects a local anesthetic to numb the skin and uses a scalpel, a sharp blade, or punch instrument to remove a small section of the rash. The biopsied area is covered by a bandage and heals within a week. 

In a lab, a pathologist who specializes in skin cells, called a dermatopathologist, examines the sample under a microscope to determine whether eczema or dermatitis is present or if a different skin condition accounts for the rash. Biopsy results take three to seven days.